1. Field of the Invention
The subject invention pertains to the field of monolithic semiconductor circuits and more particularly to a method for fabricating gallium arsenide millimeter-wave surface-oriented Schottky barrier diodes using deep mesa etching.
2. Description of the Prior Art
In the fabrication of monolithic mixers with millimeter-wave surface-oriented Schottky diodes on semi-insulating gallium arsenide substrates, it is necessary to grow or deposit on the substrate a thin lightly doped conductive gallium arsenide layer with a thicker, heavily doped layer of low resistivity underneath. The interface between the two layers must be well defined for maximum performance. This has been achieved in the prior art by incorporating an epitaxially grown or ion implanted n layer 0.1-0.2 .mu.m thick over an epitaxially grown or ion implanted n.sup.+ layer several microns thick. The n.sup.+ layer is profiled to provide a small pocket of conducting gallium arsenide in the high resistivity wafer, with the Schottky junction contact on the n layer close to the edge of this region. In order to provide isolation between the Schottky barrier junction and the n.sup.+ conductive region, and between adjacent devices on a wafer, either proton bombardment or shallow mesa etching has been used to isolate the conductive areas. In the process of proton bombardment, the diode conducting area is isolated by depositing a layer of silicon dioxide over the device areas to be protected and converting the unprotected epitaxial layers to high resistivity material. It is a significant limitation of proton bombardment that the thickness of the n.sup.+ layer must be limited to less than 3.5 .mu.m, since few implanters have energies above 400 keV, which limits the depth of proton penetration in gallium arsenide accordingly. For optimum performance, thicker n.sup.+ layers are needed in order to reduce series resistance and improve the cut-off frequency. (One skin depth at 35 GHz in n.sup.+ gallium arsenide is 5.9 .mu.m.)
Shallow mesa etching (less than 3.5 .mu.m) has also been used to provide isolation. In the etching process, the highly conducting heavily doped n.sup.+ region of gallium arsenide is removed under the Schottky barrier bonding path to provide insulation between the Schottky contact and the ohmic contact. However, only a relatively shallow region has heretofore been etched because of the difficulty in providing electrical contact over the step coverage required by the formation of a deep mesa region for isolating the ohmic contact.
A further problem that occurs with deep mesas is the difficulty of maintaining precise delineation of the fine lines required for millimeter wave devices using conventional photolithographic processes, because of the limited depth of field of the mask projection aligner.
The present invention provides a method for fabricating gallium arsenide millimeter-wave mixer diodes using deep mesa (4-7 .mu.m) structures. The method provides excellent step coverage of deposited films and accurate photolithographic reproduction of fine lines thus permitting the use of a relatively thick n.sup.+ layer with improved low series resistance and reduced parasitic capacitance.